Aluminum alloy



I Patented July 14, 1942 ALUMINUM ALLOY Walter Bonsack, South Euclid,Ohio, assignor to The National smelting Company, Cleveland, Ohio, aeorporationof Ohio No Drawing.

, This application is a continuation-in-part of my copending applicationSerial No. 389,020, filed April 17, 1941. The invention relates toalloys, and particularly to aluminum base alloys suitable for castingand. working, and having high strength at ordinary and elevatedtemperatures.

It is an object of this invention to produce alloys having relativelyhigh elongation and relatively high tensile strength.

It is a further object of this invention to provide a relatively lightalloy which may be easily east and machined, which may be used atelevated temperatures without a rapid deterioration of desirableproperties, and which may be readily treated with anodic treatment togive excellent lustre and finish.

It is astill further object of this invention to provide an alloyhavinga relatively high proportional limit and relatively high fatiguestrength, and in which these properties may be obtained without heattreatment.

When magnesium and zinc are added to aluminum in the proper proportions,a ternary compound of aluminum, magnesium and zinc is formed, whichcompound is soluble in solid solution in the aluminum. The presence ofthis compound in a relatively small amount greatly improves thecharacteristics of aluminum and produces an alloy having high strengthcombinedwith high ductility, good casting, rolling, extruding andforging properties, and good color. In

calculating the amount of magnesium and-zinc that should be present inthe aluminum alloy to form the desired percentage of ternary compound,only magnesium which is not combined with silicon is to be calculated,as it is only such magnesium that is available to combine with zinc andaluminum to form the tertiary compound.

The ternary compound is said by some investigators to have a compositionhaving substantially the formula AhMg-zzna, and other investigators haveconsidered the formula for the ternary compound ms being AlzMgzZm. Itwill be seen-that the ariiounts of magnesium and zinc relative to eachother are quite similar in, both formulae.

The magnesium and zinc should be present in about the proportionnecessary to form the ternary compound of either formulae or, preferably, the ratio of the-magnesium to zinc in the alloy should be betweenthe ratios in the formulae. I Y.

An excess of zinc, over and above that. which cooperates with magnesiumand aluminum .to

form a'ternary compound according to the above formula having thegreatest proportion of-zinc, increases the brittleness and decreases theduetility of the alloy. For this. reason it is undesire able that zincbe present in quantities substantially greater than the amount to reactto form cut on the rich side to prevent the silicon from.

Application February 20, 1942, Serial No. 431,682

9 canes- (o1. IS-446i aluminum. obtained when the magnesium and the zincare proportioned so that the ratio of magnesium (uncombined with anysilicon) to zinc is about equal to the'ratio represented by the formula.

AhMgsZna, or somewhat larger as represented by.

alloy sluggish, decreasing castability.

The improved aluminum alloys may hav the ternary compound of aluminum,zinc and magnesium present in an amount ranging from about 2% to 20%.the preferred range being between about 3% and 15%. At room temperaturethe ternary compound goes into solid solution in aluminum alloys in anamount of about 2%. The percentage in solid solution increases at hightemperatures and decreases upon cooling, the

excess precipitating out. Aluminum alloys containing the ternarycompound may, therefore, be

advantageously heat treated to improve their properties.

A small amount of silicon is usually present in aluminum alloys and upto 1.5% silicon may be present in the alloys of the present invention.

If the alloys are to be rolled or worked the silicon should be low, fromabout the minimum obtainable quantity of .04% or .05% to about 3%.

More than 17% is frequently desirable in casting alloys. Siliconcombines with magnesium in preference to most elements, each part byweight of silicon combining with about 1.75 parts, by weight, ofmagnesium to form MgzSi. At least suflicient magnesium. is thereforeadded to the alloy to combine with. the silicon uncombined with anycalcium to form Mg'aSi, and in'addition to combine with all the zinc andform the ternary compound according to the formula AlzMzaznz. MgaSi is imore stable than the ternary compound above mentioned and" may be.maintained in solid solution in aluminum alloys in an amount up'toabout1.85%,"which is. the quantity of M8231 present if the silicon is presentin the alloy, and acts as a hardener which is sometimes desirable inconjunctionwith the ternary compound. Mgesi does not. however, make asefficient useof the magnesium as does the above mentioned ternarycompound. Therefore,.it is desirable to have the magnesium presbeingpresent in excess and taking m ium away from the ternary compound.

It has now been s h a r y mpound with 'magnesiumiand 00 containingmagnesium (over that necessary to The most desirable properties arefound that aluminum alloys limit of the alloy. It also have themanganese sired in a given alloy combine with silicon) and zinc in theproportions to form a ternary compound are greatly improved byincorporating in the alloy .05% to 1.5% manganese, .05% to 1.5%chromium, .1% or .2% to 1.5% copper and, preferably, one or more of thegrain refining elements, such as those selected from the groupconsisting of titanium, columbium, zirconium, boron, tungsten,molybdenum, tantalum and vanadium. The grain refiners should be presentin a total amount of .005% to and the copper, manganese and chromiumshould preferably total less than 4%. The alloy may containfrom about.1% to about 1.5% of iron.

Manganese, although it decreases the tensile strength and elongation tosome degree, increases the yield strength, hardness and proportionalmakes the alloy more corrosion resistant. Alloys containing manganesemay be readily heat treated or age hardened to give somewhat superiorproperties, but very desirable properties are obtainable when castingsare simply aged at room temperature, or when quenchedffrom the mold andaged.

Manganese is a very eiiective element in the alloy and desirableimprovements are noted when about .1% or even a little less, such as.05%, is present in the alloy. The preferred properties are obtainedwith about .2% to about .5% or .8.% manganese, and in some cases it isdesirable to present in amounts as great as about 1%,or even 1.5%.

Chromium is a particularly eiiective alloying element in the alloy ofthepresent invention. Although it'does not appear to improve theproportional limit and yield strength of the alloy quite as much as doesmanganese, it increases the elongation and further increases corrosionresistance. It is, therefore, particularly advantageous that bothchromium and manganese be present. As little as .05% or .1% chromium,particularly with manganese, is effective in improving the propertiesof. the alloy, but .2% or .3% to about 1%, or even 1.5%, is desirable.When manganese is also present, the total of manganese and chromiumshould preferably be between about .3% and 2.5% of the alloy.

Copper functions in a somewhat diflerent manner than domanganese andchromium. Due to the fact that copper is considerably more soluble inaluminum at. high temperatures than at low temperatures, copper acts asa precipitation hardening ingredient, so an alloy containing copper isbenefited more by solution heat treatment.

Small proportions of copper even without solution treatment improves theproportional limit and yield strength of the alloy. As little as .1% ofcopper noticeably improves the alloy, but it is preferable, especiallyin alloys to be solution heat treated, that about .2% to about 1.5% ofpresence of from .4% or copper be present. The .5% to 1.5% of copperpermits a reduction in the amount of temarycompound so that a desirablealloy is produced with as little as 1% or the ternary compound.

The quantity ofmanganese and chromium dealso depends somewhat upon thequantity of copper present and upon the amount of ternary compound, agiven hardness and tensile strength often being obtainable either with arelatively large amount o'f strength-improving metalsland a relativelysmall amount of ternary compound, or with a relatively small amount ofsuch metals and arelatively large amount of magnesium and zinc in theproportions of a ternary compound.

An alloy containing 2%, or even as little as 1%, of the ternary compoundmay be used for casting purposes. The castability, however, is improvedwith an increase in the amount of ternary compound and it is, therefore,preferred to have a larger percentage of the ternary compound present,such as 4% to 8% or 9% for casting purposes, 9% ternary compoundcontaining about 5.4% zinc. When the casting is more or less intricatelyshaped, still greater percentages, such as 10% to 12%, of the ternarycompound may be present. For alloys to be forged or shaped aftercasting, the ternary compound should be present in the lower ranges,such as 1% to 6% or so, as the metal is less hard with the lowerpercentages of the ternary compound.

A larger proportion of the ternary compound may be present in alloyswhich are to be given a so-called solution treatment than in alloys tobe given only an aging treatment, or those to be quenched from thecasting mold and aged at relatively low temperatures. Thus, thedesirable properties of the solution heat treated alloys may be obtainedwhen they contain the ternary compound in amounts up to 12% or so,whereas less of the ternary compound, such as 4% to 6%, is preferred inalloys which are quenched upon removal from the mold and heat treated ata low temperature, or aged at room temperature'.

It has generally been considered that aluminum alloys of magnesiumcontaining iron much above the impurity value in commercial aluminum areof little commercial value, but it has also now been found that an alloycontaining the magnesium, zinc and silicon proportioned as hereindescribed, and also containing manganese, chromium and a grain refiningmetal such as is set forth in the above group, is improved by thepresence of iron in suitable proportion.

Iron in suitable amounts further increases the hardness and tensilestrength of the alloy without decreasing its ductility a substantialamount. A small amount of iron thus permits one to obtain the propertiesdesired with a smaller amount of magnesium and zinc. Iron also improvescast ability. These alloys containing iron may be readily heat treatedor age hardened to give somewhat superior properties, but the iron incombination with manganese, chromium and the ternary elements in theabove proportion is also outstanding in that almost as desirableproperties are obtained when castings are aged at room temperaturewithout a heat treatment or quenching.

Iron has generally been considered to crystallize in large platelikecrystals, which weaken the alloy. Iron in the presence of the ternarycompound appears to crystallize in finely dispersed form and the ternarycompound also seems to be dispersed, thus producing a highly desirablealloy.

age of iron in the alloys, the percentages of manganese and chromium maybe reduced considerably. If both manganese and chromium are present insubstantial amounts and it workability, ductility and corrosionresistance are not to be impaired, the iron should be low, 1. e., about0.1% or less to about ..6% or .7% of the alloy. If yield strength andhigh tensile strength are most important and ductility less important,the amount 01' iron may be greater.

The grain refining elements are particularly desirable in an aluminumalloy containing iron, copper, manganese, chromium and the ternarycompound. Although the iron itself improves the properties of the alloy,the manganese, chromium, copper and grain refining elements exert astill further improvement independently of iron.

The aluminum alloys of the present invention containing magnesium,uncombined with silicon, and zinc in the proportion of a ternarycompound, when cast in molds of a design such that chilling takes placesubstantially simultaneously in thevarious portions, of the casting,solidify without the use of grain refining agents to form good castings.However, it has been found that certain grain refining elementssubstantially'improve the properties of the aluminum alloys containingthe ternary compound, iron, copper, manganese and chromium. This isespecially true when the metal is cast inmolds of more or less intricateshape where the chilling may not be so uniform throughout the casting.

The grain refiners which I have found exert greatest improvement in theproperties of the alloy are members of the-group consisting of boron inthe amount of .005% to .1%, zirconium in the amount of .0l% to .5%,tungsten in the amount of .0l% to .5%, molybdenum in the amount of .01%to .5%, vanadium in the amount of .01% to 5%, titanium in the amount of.05%

to .5%, columbium in the amount of .01% to .5% and tantalumin the amountof 05% to .5%. These grain refining elements should preferably bepresent in a total amount or from 005% to .5% and it is frequentlydesirable to have more than one of these elements present in a givenalloy.

While the grain refiners in the abovegroup are desirable in the alloysof the present invention, not all of the grain refiners aflecttheproperties in the same way. The particular refiner or group of refinersselected in any given instance depends upon the particular conditionwhich must be satisfied. The grain refiners selected from the groupconsisting of titanium, tungsten, molybdenum, zirconium and vanadium,and especially tungsten and molybdenum, improve both the strength andthe elongation of the castings. Titanium being readily available isfrequently used, but when special properties of the fabricated articleare important it is desirable to select the grain refiner that is mostsuitable for such properties. The grain refiners boron, co-

lumbium and tantalum may advantageously be used where appearance, finishand corrosion resistance are important. 1

The abevedescribed hardening elements, manganese and chromium,substantially decrease the hot shortness, improve the properties of thealloy and assist in maintaining the improved properties at hightemperatures such as are encoun-' tered in internal combustion engines.The above grain refining elements, particularly members of the groupconsisting of zirconium, tungsten, mo-

lybdenum, vanadium and titanium, also have this property particularlywhen present in larger amounts, such as .2% or .3% or so. It may,therefore, be desirable to have up to 5% or so of these latter elementspresent.

The following examples illustrate the alloys of the present invention.

Example 1 An aluminum base alloy containing 6% of the ternary compoundbased on the formula- AhMg'zZns, about .3% manganese, about -.25%chromium, about .2% copper, about .2% titanium, about .2% silicon, andabout .6% iron, was prepared and test bars were chill cast from thisalloy. Part of the test bars were air cooled from the mold and part werequenched, and both kinds of test bars were aged seven days at roomtemperature. The respective tensile strengths of the aged and quenchedtest bars were 38,600 lbs/sq.

, 42,000 lbs/sq. in., the respective proportional limits were 19,300lbs/sq. in. and 18,100 lbs/sq. in., the respective yield strengths were32,400 lbs/sq. in. and 30,400 lbs./sq. in., the RockwellE hardness was86.9 and 83.7, respectively, and the elongation was 5.9% and 7.6%,respectively.

When the copper content was about 1% the respective tensile strengths ofthe aged and quenched test bars were 36,200 lbs/sq. in. and 37,600lbs./sq. in., the respective proportional limits were 26,900 lbs/sq. in.and 19,400 lbs/sq. in., the respective yield strengths were 33,900lbs/sq. in. and 31,800 lbs/sq. in., the Rockwell E hardness was 85.1 and82.5, respectively, and the elongation was 2.6% and 4.6%, respectively.

Example 2 Sand cast test bars were prepared from an alloy containingabout 1.72% magnesium, about 3.69% zinc, about 28% silicon, about 27%manganese, about 28% chromium, about .18% titanium, about .64% iron andabout 23% copper, with the balance substantially all aluminum and minorimpurities. 7 Some or the. bars were aged seven The second group had atensile strength of 32,200 lbs/sq. in., a proportional limit of 16,300lbs/sq. in., a yield strength of 27,600 lbs./sq.in..' a Rockwell Ehardness of 77.6, and an elongation of 2%. a l

The third group had a tensile strength of 30,300 '1bs./sq. in.,. aproportional limit of 12,900 lbs/sq. in., a yield strength or 20,900lbs/sq. in., a Rockwell E hardness 0170.1, and an elongation of 4.3%.

Since the molecular proportion of zinc is never more than the molecularproportion or the relathe like.

tively light magnesium in the ternary compound, iii-is seen that inaddition to high strength the alloys are light in weight and are.therefore, especially adapted to aircraft construction and This isparticularly true when the quantity of ternary compound is suilicientlylow so that the alloy may be drawn or rolled into structural members.

If the alloy contains uncombined silicon, about 1.75% magnesium isrequired to combine with each percent of uncombined silicon to formmagnesium silicide (MgzSi) before any ternary compound will be formed.For example, if 2% of the ternary compound on the basis of AlzMgaZnc Abe desired in an alloy having .3% silicon, the

ing .7% free silicon and AhMgrZne would be about 7% and 12%,respectively. The alloys described herein include aluminum, magnesiumand zinc, the magnesium, uncombined with silicon,being proportioned tothe zinc in the ranges of the formulae given for the ternary compound.The proportions for the formation of the ternary compound in the alloyexist when the magnesium is about 35% to 45% of the zinc content plus175% of the silicon content. Most desirable properties may be obtainedwhen the magnesium (uncombined with silicon) is in the lower portion ofthis range, or about 35% to 40% of-the zinc.

In the above examples of alloys of the present invention it is to benoted that excellent tensile strength and hardness are obtainable in arelatively short time by aging at room temperature. A very astoundingfact has been discovered, however, in connection with these alloys,namely, that the tensile strength may increase up to approximately ofits initial value by aging at room temperature for relatively longperiods of time, such as a few months. The same improvement in tensilestrength can, of course, be obtained relatively quickly by aging attemperatures above room temperature.

To obtain properties even of the same order in use, one has to resort toa solution and aging heat treatment, whereas in alloys of the presentinvention it is not necessary to solution heat treat for improvement inproperties.

The alloys of the present invention have good fatigue and tensilestrength and a relatively high proportional limit, even at relativelyhigh temof magnitude in aluminum base alloys commonly peratures; theyhave to be heat treated if it is desired toimprove and modify theirproperties, although such heat treatment is not required; and they havesufiicient ductility and hardness so. that they can be rolled or formedinto sheets, rods, wire, structural shapes, castings, machine parts,etc. These alloys have a desirable color and are suitable for many uses,among them being the production of castings which are shaped or formedto some extent aftercasting. The alloys having the-lower percentages ofternary comvpound may'even be forged at room temperature and are'thususeful for many special purposes.

As pointed'out above, grain refiners are usually desirable in alloys ofthe present invention; However, it has been found that the alloys of thepresent inventionhave relatively high strength, high proportional limitand ductility without solution heat treatment, even in the absence oilthe grain refiners. The alloys may be cast, rolled,

forged or otherwise shaped with or without grain refiners.

It is to be understood that, in considering the.

amount of zinc and magnesium to add to aluminum alloys to form theternary compound of aluminum, magnesium and zinc in the alloy, suchmagnesium as is necessary to combine with the I defined in the appendedclaims.

What I claim is:

1. An aluminum alloy containing magnesium, zinc, about .1% to 1.5% iron,about .05% to1.5% manganese, about .05% to 1.5% chromium, about .1% to1.5% copper, silicon in an amount up to 1.5%, and one or more grainrefining metals, with the balance substantially all aluminum and minorimpurities, the amount oi'zinc in the alloy being about .6% to 7.2%, andthe amount of magnesium in the alloy uncombined with silicon being about35% to 45% of thezinc content, the total magnesium being within therange of about 2. An aluminum alloy containing magnesium, zinc, about.1% to 1.5% iron, about .05% to 1.5% manganese, about .05% to 1.5%chromium, about .1% to 1.5% copper, silicon in an amount up to 1.5%, andone or more grain refining metals in a total amount of 005% to about.5%, with the balance substantially all aluminum and minor impurities,the amount of zinc in the alloy being about .6% to 7.2%, and the amountof magnesium in the alloy uncombined with silicon being about 35% to 45%of the zinc content, the total magnesium being within the range of about.4% to 7%. r

3. The alloy of claim 1 in which the zinc content is about 1.2% to 6%and the magnesium content is within the range of about .5% to 6%.

4. The alloy of claim 1 in which the zinc content is about 1.2% to 4.8%,the magnesium content is within the range of about .5% to 5%, and thecopper content is within the range of .2% to 1.5%.

5. The alloy set forth in claim 2 in which the copper is present in theamount of .1% to .5%.

6. The alloy set forth in claim 2 in which titanium is present in theamount of .05% to .5%.

7. The alloy set forth in claim 2 in which zirconium is present in theamount of .01% to .5%.

8. The alloy set forth in claim 2 in which molybdenum is present in theamount of .01% to 9. An aluminum alloy containing magnesium, zinc,about.1% to 1.5% iron. about .05% to 1.5% manganese, about .05% to 1.5%chromium, about .1% to 1.5% copper, and silicon in an amount up to 1.5%,with the balance substantially all aluminum and minor impurities, theamount of zinc in the alloy being about .6% to 7.2%, and the amount ofmagnesium in the alloy uncombined with silicon being about 35% to 45% ofthe zinc content, the total magnesium being within the range of about.4% to 7%.

WALTER BON SACK.

